You might be able to detect some degree of movement using the Flex Sensor's available from various sources? It would probably take some amplification and filtering (signal processing) to improve the integrity of the output but I would think it could be done.

How intrusive is it allowed to be? You mention mounting it on glasses so I'm assuming it can touch the user's brows etc (as opposed to trying to do it with some sort of computer vision).

Cheers,

ripred